The equation that describes the energy output required to maintain the body's complex functions is simple and familiar. If the calories we ingest exceed the calories we expend in our day-to-day routine, then excessive weight gain results. However, the mechanisms associated with being overweight or obese are anything but simple. There is increasing evidence that what we eat is influenced by the availability of healthy foods and spaces for physical activity in the places where we live, work, learn, and play.